Nonaqueous electrolyte secondary battery separator and use thereof

ABSTRACT

A nonaqueous electrolyte secondary battery separator, which includes a porous film containing a polyolefin-based resin as a main component, has a difference of not more than  2.5  between (a) a white index measured on a surface of the porous film which has not been irradiated with ultraviolet light having  255  W/m 2  and (b) a white index measured on the surface of the porous film which has been irradiated, for  75  hours, with the ultraviolet light having  255  W/m 2 .

This Nonprovisional application claims priority under 35 U.S.C. §119 onPatent Application No. 2015-233930 filed in Japan on Nov. 30, 2015, theentire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a separator for a nonaqueouselectrolyte secondary battery (hereinafter referred to as a “nonaqueouselectrolyte secondary battery separator”) and use of the nonaqueouselectrolyte secondary battery separator. More specifically, the presentinvention relates to a nonaqueous electrolyte secondary batteryseparator, a laminated separator for a nonaqueous electrolyte secondarybattery (hereinafter referred to as a “nonaqueous electrolyte secondarybattery laminated separator”) including the nonaqueous electrolytesecondary battery separator, and a nonaqueous electrolyte secondarybattery.

BACKGROUND ART

Nonaqueous electrolyte secondary batteries, especially lithium-ionsecondary batteries, each have a high energy density. Thus, suchnonaqueous electrolyte secondary batteries have been widely used asbatteries for use in, for example, a personal computer, a mobile phone,and a portable information terminal, and also have recently beendeveloped as on-vehicle batteries.

Conventionally, a microporous film that contains polyolefin as a maincomponent has been used as a nonaqueous electrolyte secondary batteryseparator (hereinafter may also foe referred to as merely a “separator”)included in a nonaqueous electrolyte secondary battery such as alithium-ion secondary battery (Patent Literature 1).

The microporous film has therein pores connected to one another, andallows a liquid containing ions to pass th ere th rough from one surfaceto the other. This microporous film is thus suitable as a batteryseparator member that exchanges ions between a cathode and an anode.

Meanwhile, recent widespread use of nonaqueous electrolyte secondarybatteries has created a demand For longer-life nonaqueous electrolytesecondary batteries.

CITATION LIST Patent Literature

[Patent Literature 1]

Japanese Patent Application Publication Tokukai No, 2003-105120(Publication Date: Apr. 9, 2003)

SUMMARY OF INVENTION Technical Problem

From the viewpoint of meeting such, a demand for battery life extension,conventional nonaqueous electrolyte secondary batteries including anonaqueous electrolyte secondary battery that includes the separatordisclosed in Patent Literature 1 unfortunately cannot be said to have asufficient long-term property (cycle characteristic). This creates ademand for a nonaqueous electrolyte secondary battery that continues toshow a high discharge capacity maintaining ratio even after charge anddischarge is repeated many times.

The present invention has been made in view of the problem, and anobject of the present invention is to provide a nonaqueous electrolytesecondary battery that is so excellent in cycle characteristic as tosufficiently meet a demand for battery life extension.

Solution to Problem

In order to attain the object, the inventor of the present inventionmade a diligent study and finally accomplished the present invention byfinding (i) that an amount by which a white index (hereinafter may alsobe referred to as “WI”) of a separator changes before and after theseparator is irradiated with ultraviolet light under a given conditionis correlated with a cycle characteristic of a nonaqueous electrolytesecondary battery and (ii) that the amount which has a value that is notmore than a given value makes it possible to provide a nonaqueouselectrolyte secondary battery that can attain the object.

In order to attain the object, a nonaqueous electrolyte secondarybattery separator in accordance with an aspect of the present inventionincludes a porous film containing a polyolefin-based resin as a maincomponent, the nonaqueous electrolyte secondary battery separator havingΔWI of not more than 2.5, ΔWI being defined by the following Equation(1):

ΔWI=WI ₁ −WI ₀  Equation (1)

where WI is a white index defined by American Standard Test Method(ASTM) E313, WI₀ is WI that is measured, by use of a spectrocolorimeter,on a surface of the porous film which has not been irradiated withultraviolet light having 255 W/m², and WI₁ is WI that is measured, byuse of the spectrocolorimeter, on the surface of the porous film whichhas been irradiated, for 75 hours, with the ultraviolet light having 255W/m².

A nonaqueous electrolyte secondary battery laminated separator inaccordance with an aspect of the present invention preferably includes anonaqueous electrolyte secondary battery separator mentioned above and aporous layer.

A nonaqueous, electrolyte secondary battery member in accordance with anaspect of the present invention includes: a cathode; a nonaqueouselectrolyte secondary battery separator mentioned above; and an anode,the cathode, the nonaqueous electrolyte secondary battery separator, andthe anode being provided in this order.

A nonaqueous electrolyte secondary battery in accordance with, an aspectof the present invention includes a nonaqueous electrolyte secondarybattery separator mentioned above or a nonaqueous electrolyte secondarybattery laminated separator mentioned above.

Advantageous Effects of Invention

A nonaqueous electrolyte secondary battery separator in accordance withan aspect of the present invention has ΔWI of not more than 2.5, ΔWIbeing defined by Equation (1). Thus, the nonaqueous electrolytesecondary battery separator contains, in a smaller amount, an oxide thatis generated during production of a porous film and causes a battery tohave a shorter life. This allows an aspect of the present invention toyield an effect of providing a nonaqueous electrolyte secondary batterythat has an excellent cycle characteristic.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically illustrates how a sheet made of a resin is formedby extruding and rolling the resin.

DESCRIPTION OF EMBODIMENTS

The following description will specifically discuss an embodiment of thepresent invention. Note that “A to B” herein means “not less/lower thanA and not more/higher than B”.

<Nonaqueous Electrolyte Secondary Battery Separator>

A nonaqueous electrolyte secondary battery separator in accordance withan embodiment of the present invention includes a porous film containinga polyolefin-based resin as a main component, the nonaqueous electrolytesecondary battery separator having ΔWI of not more than 2.5, ΔWI beingdefined by the following Equation (1):

ΔWI=WI ₁ −WI ₀  Equation (1)

where WI is a white index defined by American Standard Test Method(ASTM) E313, WI₀ is WI that is measured, by use of a spectrocolorimeter,on a surface of the porous film which has not been irradiated withultraviolet light having 255 W/m², and WI₁ is WI that is measured, byuse of the spectrocolorimeter, on the surface of the porous film whichhas been irradiated, for 75 hours, with the ultraviolet light having 255W/m².

(1) Porous Film

A porous film of an embodiment of the present invention contains apolyolefin-based resin as a main component, and is preferably amicroporous film. That is, the porous film preferably (i) has thereinpores connected to one another and (ii) contains, as a main component, apolyolefin-based resin that can pass therethrough a gas and/or a liquidfrom one surface to the other surface. The porous film can include asingle layer or a plurality of layers.

The porous film that contains a polyolefin-based resin as a maincomponent refers to a porous film that contains a polyolefin-based resincomponent, in an amount typically of not lower than 50% by volume,preferably of not lower than 90% by volume, and more preferably of notlower than 95% by volume, relative to the entire porous film. Thepolyolefin-based resin contained in the porous film preferably containsa high molecular weight component having a weight-average molecularweight of 5×10⁵ to 15×10⁶. The porous film which contains, as the polyolefin-based resin, especially a polyolefin-based resin having aweight-average molecular weight of 1,000,000 is more preferable. This isbecause such a porous film allows an increase in strength of (i) theentire porous film, i.e., the entire nonaqueous electrolyte secondarybattery separator and (ii) a nonaqueous electrolyte secondary batterylaminated separator that includes the porous film and a porous layer(described later).

Examples of the polyolefin-based resin include high molecular weighthomopolymers (such as polyethylene, poly propylene, and polybutene) andcopolymers (such as an ethylene-propylene copolymer) produced by(co)polymerizing a monomer such as ethylene, propylene, 1-butene,4-methyl-1-pentenes or 1-hexene. The porous film is a layer containingone kind of the polyolefin-based resins and/or a layer containing two ormore kinds of the polyolefin-based resins. Among the above examples, ahigh molecular weight polyethylene-based resin containing ethylene as amain component is particularly preferable in terms of its capability toprevent (shut down) a flow of excessively large current at a lowertemperature. Note that the porous film can contain other componentdifferent from a polyolefin-based resin, provided that the othercomponent does not impair a function of the layer.

Examples of the polyethylene-based resin include low-density polyethylene, high-density polyethylene, linear polyethylene (anethylene-α-olefin copolymer), and ultra-high molecular weightpolyethylene having a weight-average molecular weight of not less than1,000,000. Among these polyethylenes, ultra-high molecular weightpolyethylene having a weight-average molecular weight of not less than1,000,000 is still more preferable.

(2) Nonaqueous Electrolyte Secondary Battery Separator

The separator has a film thickness preferably of 4 μm to 40 μm, morepreferably of 5 μm to 30 μm, and still more preferably of 6 μm to 15 μm.

The separator only needs to have a mass per unit area which mass isdetermined as appropriate in view of a strength, a film thickness, aweight, and handleability of the separator. Note, however, that theseparator has a mass per unit area preferably of 4 g/m² to 20 g/m², morepreferably of 4 g/m² to 12 g/m², and still more preferably of 5 g/m² to10 g/m² so as to allow a nonaqueous electrolyte secondary batteryincluding the separator to have a higher weight energy density and ahigher volume energy density.

The separator has a Gurley air permeability preferably of 30 sec/100 mLto 500 sec/100 ml and more preferably of SO sec/100 mL to 300 sec/100mL. The separator which has a Gurley air permeability falling within theabove range makes it possible to obtain sufficient ion permeability.

The separator has a porosity preferably of 20% by Volume to 80% byvolume and more preferably of 30% by volume to 75% by volume so as to(i) retain a larger amount of an electrolyte and (ii) obtain a functionof preventing (shutting down) a flow of an excessively large electriccurrent at a lower temperature without fail. Further, in order to obtainsufficient ion permeability and prevent particle s from entering acathode and/or an anode, the separator has pores having a pore sizepreferably of not larger than 0.3 μm and more preferably of not largerthan 0.14 μm.

The nonaqueous electrolyte secondary battery separator in accordancewith an embodiment of the present invention has ΔWI of not more than2,5, ΔWI being defined by Equation (1). WI is an indicator of a colortone (whiteness) of a sample, and is used to indicate a fadingcharacteristic of a dye or a degree of oxidation degradation intransparent or white resin that is being processed. Higher WI means ahigher degree of whiteness,

A separator can be produced by, for example, (i) a method in which aporous film is obtained by adding a pore forming agent such as a filleror a plasticizer to a resin such as polyolefin so as to form a sheet,thereafter removing the pore forming agent by use of an appropriatesolvent, and stretching the sheet from which the pore forming agent hasbeen removed, or (ii) a method in which a porous film is obtained byadding a pore forming agent to a resin such as polyolefin so as to forma sheet, thereafter stretching the sheet, and removing the pore formingagent from the stretched sheet.

The sheet is formed by, for example, extruding, from a T-die or thelike, the resin containing the pore forming agent, and then causing apair of rollers to roll the resin so as to form the resin into a thinfilm. FIG. 1 schematically illustrates how a sheet made of a resin isformed by extruding and rolling the resin. In FIG. 1, a referencenumeral 1 indicates a resin of which the separator is made, a referencenumeral 2 indicates a T-die, a reference numeral 3 indicates a roller,and a reference numeral 4 indicates a distance between the T-die 2 andthe roller 3.

For example, in Comparative Production Examples 1and 2 (describedlater), the sheet was formed under a condition where (i) the resin 1 hadrespective temperatures of 253° C. and 252° C. immediately before beingextruded from the T-die 2, and (ii) the roller 3 had a surfacetemperature of 150′C, Note that this condition is a well-known conditionthat can be typically employed to produce a separator. During formationof the sheet, the resin 1 which has a high temperature is exposed to airduring a period in which the resin 1 is extruded from the T-die 2 andthen brought into contact with the roller 3. This brings the resin 1into contact with oxygen, so that an oxide of the resin 1 is generated.The oxide may cause a side reaction during charge and discharge of abattery and consequently shortens a life of the battery. Thus, theporous film preferably contains the oxide in an amount as small aspossible.

Meanwhile, the oxide loses color in response to ultraviolet lightirradiation. In view of this, the porous film which more greatly changesin WI in a case, where (i) WI measured on a surface of the porous filmwhich has not been irradiated with ultraviolet light and (ii) WImeasured on the surface of the porous film which has been irradiatedwith ultraviolet light are compared can foe said to contain, the oxidein a greater amount.

The inventor of the present invention carried out a study whileconsidering that production of a separator that less changes in WI,i.e., a separator that contains the oxide in a smaller amount, makes itpossible to achieve battery life extension while preventing, the oxidefrom influencing a cycle characteristic of a battery. The study made itclear that the porous film, which, has ΔWI of not more than 2.5, ΔWIbeing defined by Equation (1), allows a nonaqueous electrolyte secondarybattery including a. separator, which is the porous film, to have anexcellent cycle characteristic.

There had conventionally not existed knowledge (i) that an amount ofchange in WI and a cycle characteristic of a nonaqueous electrolytesecondary battery are correlated with each other and (ii) that anonaqueous electrolyte secondary battery having an excellent cyclecharacteristic can be provided by adjusting ΔWI to not more than 2.5.This knowledge was found out by the present invention for the firsttime.

As defined by Equation (1), ΔWI is a difference between (a) WI (WI₀)measured, by use of a spectrocolorimeter, on the surface of the porousfilm which has not been irradiated with ultraviolet light having anintensity of 255 W/m² and (b) WI (WI₁) measured, by use of thespectrocolorimeter, on the surface of the porous film which has beenirradiated, for 75 hours, with the ultraviolet light having an intensityof 255 W/m². WI0 is WI measured on the surface of the porous film whichhas not been irradiated with the ultraviolet, light of 255 W/m² (i.e.,WI measured on the surface of the porous film before 255 W/m²ultraviolet light irradiation starts to be carried out with respect tothe porous film).

The spectrocolorimeter is, for example, preferably an integrating-spherespectrocolorimeter, which can easily and accurately measure WI. Theintegrating-sphere spectrocolorimeter is a device for carrying outoptical spectrometric measurement by (i) irradiating a sample with lightof a xenon lamp and (ii) causing an integrating sphere that covers thevicinity of an irradiated portion to collect, in a light receivingsection, light reflected from the sample. The integrating-spherespectrocolorimeter allows measurement of various optical parameters.Note, however, that the spectrocolorimeter is not particularly limitedto the integrating-sphere spectrocolorimeter, and can be anyspectrocolorimeter that can measure WI.

The “surface of the porous film” refers to a part of the porous filmwhich part receives light emitted from the spectrocolorimeter. Themeasurement of WI on the surface of the porous film by use of thespectrocolorimeter only needs to be carried out in accordance with aninstruction manual of a spectrocolorimeter to be used, and does not needto be carried out by any particular method. Note, however, that in orderthat light reflected by the porous film can be easily collected in alight receiving section of the spectrocolorimeter it is preferable to,for example, irradiate the porous film which is placed on black paper.

The 255 W/m² ultraviolet, light irradiation is preferably carried out byuse of a device that can continuously carry out ultraviolet, lightirradiation. For example, it is possible to use a fade meter or aweatherometer (e.g., sunshine weather meter S80 manufactured by SugaTest Instruments Co., Ltd.) defined by JIS B 7753. in a case where thesunshine weather meter S80 is used, the ultraviolet light irradiation iscarried out with respect to a test piece for 75 hours by use of asunshine carbon arc light lamp (four pairs of ultra long life carbon),which is set to have a discharge voltage of 50 V and a discharge currentof 60 A., and under conditions of a black panel temperature of 60° C.and a relative humidity of 50%.

The test machine mentioned above can continuously expose a sample toultraviolet light by rotation on an ultraviolet light lamp of a metalplate to which the sample has been attached. A weatherometer can (i)intermittently spray a test piece with water while irradiating the testpiece with light that is emitted from an artificial light source andresembles natural sunlight, or (ii) repeat (a) a state of irradiation ofa test piece with light that is emitted from an artificial light sourceand resembles natural sunlight and (b) a state of darkness caused byturning off the artificial light source, and spray a back surface of thetest piece with cold water in the state of darkness. As such, theweatherometer can simulate a rainy weather (high humidity) condition.Note, however, that according to the present invention, it is onlynecessary to cause the porous film to lose color to an extent that adifference between WI₀ and WI₁ can be calculated, and thus it is notparticularly necessary to simulate a rainy weather (high humidity)condition.

As shown in Examples (described later), in a case where the porous filmhas ΔWI of not more than 2.5, ΔWI being defined by Equation (1), anonaqueous electrolyte secondary battery produced by use of the porousfilm has a high discharge capacity maintaining ratio of more than 70.0%even after being subjected to 180 cycles of charge and discharge. Asdescribed earlier, ΔWI is correlated with an amount of an oxidecontained in the porous film, and higher ΔWI indicates that the oxide iscontained in a greater amount. Thus, smaller ΔWI is more preferable.This makes it essential that ΔWI be not more than 2.5, more preferablynot more than 2.3, and still more preferably not more than 2.2. Notethat ΔWI has a lower limit preferably of not less than −10, morepreferably of not less than −5, and most preferably of 0.

The porous film which has ΔWI of not more than 2.5 can be produced bycausing a resin which has a high temperature to be exposed to air for ashorter time during formation of a sheet. In a case where FIG. 1 istaken as an example, the porous film which has ΔWI of not more than 2.5can be produced by, for example, a method (1) in which the resin 1 isextruded from the T-die 2 at a lower temperature (extrusiontemperature), a method (2) in which the resin 1 is extruded at a higherspeed so that the resin 1 is in contact with oxygen for a shorter time,a method (3) in which the distance 4 between the T-die 2 and the roller3 is made shorter, and/or (4) the like,

In a case where (a) the fact that the resin 1 needs to be extruded at atemperature that is raised to a relatively high temperature and (b) thefact that the resin 1 which has a high temperature needs to be exposedto air for a shorter time are considered by comparison, the extrusiontemperature is preferably 200° C. to 250° C., more preferably 220° C. to249° C., and still more preferably 240° C. to 248° C. in the method (1)(described earlier). Note that the extrusion temperature refers to atemperature measured immediately before the resin is extruded from anoutlet (e.g., a T-die) of an extrusion machine, and is equal to apreset, temperature of the outlet.

The extrusion speed, which also depends on a throughput capacity of theextrusion machine and thus cannot be generalized, is preferably 1 m/minto 10 m/min, more preferably 2 m/min to 8 m/min, and still morepreferably 2.5 m/min to 5 m/min in the method (2) (described earlier).

As shown in Comparative Example 1 (described later), in a case where theextrusion temperature exceeds 250° C., there was a tendency for an oxideto be contained in the resin in a greater amount even if the extrusionspeed was increased. Thus, it is considered that the oxide can becontained in the porous film in a smaller amount by increasing theextrusion speed after setting the extrusion temperature to such apreferable temperature as described earlier.

The distance 4 is preferably as short as possible in the method (3)(described earlier), provided that, the distance does not preventmovement of the roller 3. Note that the roller 3 has a surfacetemperature preferably of 120° C. to 160° C., more preferably of 130° C.to 155° C., and still more preferably of 140° C. to 150° C.

The above description has discussed conditions under which to extrudethe resin. Note, however, that the porous film which has ΔWI of not morethan 2.5 can be effectively produced also by a method (4) in which inorder that a pore forming agent is removed from a sheet that has beenformed by being rolled by the roller 3, a surfactant is added to asolvent through which the sheet is allowed to pass. Assume, for example,that an aqueous hydrochloric acid solution is used as the solvent andthe pore forming agent (e.g., calcium carbonate) that has been dispersedin the sheet is removed by dissolving the pore forming agent in theaqueous hydrochloric acid solution. In this case, the aqueoushydrochloric acid solution to which the surfactant is added canpermeate, with a higher degree, through a polyolefin-based resin ofwhich the sheet is made. As a result, the oxide which is contained inthe resin and which is an organic matter and thus is normally insolublein an aqueous hydrochloric acid solution is more easily dissolved in theaqueous hydrochloric acid solution. This makes it possible to promoteextraction of the oxide into the aqueous hydrochloric acid solution.

As the surfactant, any of an anionic surfactant, a cationic surfactant,a non-ionic surfactant, and an amphoteric surfactant can be used. Note,however, that a non-ionic surfactant, which is less susceptible to acidand alkali, is preferably used. A cleaning liquid to which thesurfactant is added in a larger amount allows (i) the cleaning (removal)of the pore forming agent and (ii) removal of the oxide to be carriedout with higher efficiency. Note, however, that the cleaning liquid towhich the surfactant is added in a too large amount may cause thesurfactant to remain in the separator. The surfactant is added in anamount preferably of not less than 0.1% by weight and not more than 15%by weight, and more preferably of 0.1% by weight to 10% by weight,relative to 100% by weight of the cleaning liquid.

The pore forming agent is removed with higher efficiency in accordancewith an increase in temperature (cleaning temperature) of the solvent.Note, however, that an excessive increase in temperature of the solventcauses evaporation of the cleaning liquid. Thus, the solvent which, is,for example, water has a temperature preferably of not lower than 25° C.and not higher than 60° C., more preferably of not lower than 30° C. andnot higher than 55° C., and particularly preferably of not lower than35° C. and not higher than 50° C. Note that the “cleaning temperature”refers to a temperature of the cleaning liquid in which the sheet, hasbeen immersed.

The sheet which has been cleaned with the solvent can further be cleanedwith water. The cleaning with water is carried out at a water-cleaningtemperature preferably of not lower than 25° C. and not higher than 60°C., more preferably of not lower than 30° C. and not higher than 55° C.,and particularly preferably of not lower than 35° C. and not higher than50° C. This is because though a higher water-cleaning temperature allowsthe cleaning with water with higher efficiency, a too highwater-cleaning temperature causes a cleaning liquid (water) toevaporate. Note that the “water-cleaning temperature” refers to atemperature of the water in which the sheet has been immersed.

The porous film which is formed by one or more methods selected from themethods (1) through (4) (described earlier) can contain the oxide in asmaller amount. This makes it possible to produce the porous film whichhas ΔWI of not more than 2.5. Of the methods (1) through (4), adjustmentof the extrusion temperature of the resin (the method (1)) and additionof the surfactant to the solvent (the method (4)) are more preferablyused in combination to form the porous film. This is because the method(1) and the method (4) each (i) allow the conditions, under which toextrude the resin, to be easily adjusted and (ii) allow the oxide to beeffectively removed.

The filler (pore forming agent) is exemplified by but not particularlylimited to an inorganic filler that is soluble in an acid-containingaqueous solvent, an inorganic filler that is soluble in analkali-containing aqueous solvent, or an inorganic filler that issoluble in an aqueous solvent mainly composed of water.

Examples of the inorganic filler that is soluble in an aqueous acidicsolvent include calcium carbonate., magnesium carbonate, bariumcarbonate, zinc oxide, calcium oxide, aluminum hydroxide, magnesiumhydroxide, calcium hydroxide, calcium sulfate, and the like. Of theseinorganic fillers, calcium carbonate is preferable in terms of easinessto obtain a fine powder thereof at low cost,

Examples of the inorganic filler that is soluble in an aqueous alkalinesolvent include silicic acid and zinc oxide, and the like. Of theseinorganic fillers, silicic acid is preferable in terms of easiness toobtain a fine powder thereof at low cost.

Examples of the inorganic filler that is soluble in an aqueous solventmainly composed of water include calcium chloride, sodium chloride,magnesium sulfate, and the like,

The plasticizer (pore forming agent) is exemplified by but notparticularly limited to low-molecular-weight hydrocarbon such as liquidparaffin.

The nonaqueous electrolyte secondary battery separator in accordancewith an embodiment of the present invention has ΔWI of not more than2.5, and thus contains the oxide in a smaller amount than aconventionally known separator. Thus, the nonaqueous electrolytesecondary battery separator allows a side reaction to be less likely tooccur during charge and discharge of a nonaqueous electrolyte secondarybattery, and consequently makes it possible to provide a nonaqueouselectrolyte secondary battery that has an excellent cyclecharacteristic.

<Nonaqueous Electrolyte Secondary Battery Laminated Separators>

The nonaqueous electrolyte secondary battery separator in accordancewith an embodiment of the present invention can also include publiclyknown porous layer(s) such as an adhesive layer, a heat-resistant layer,and/or a protective layer. A separator including the nonaqueouselectrolyte secondary battery separator and a porous layer is hereinreferred to as a nonaqueous electrolyte secondary battery laminatedseparator (hereinafter may be referred to as a “laminated separator”).

The separator is more preferably subjected to a hydrophilizationtreatment before the porous layer is formed, i.e., before the separatoris coated with a coating solution. The separator which is subjected tothe hydrophilization treatment is more easily coated with the coatingsolution. This makes it possible to form the porous layer which is moreuniform. The hydrophilization treatment is effective in a case wherewater accounts for a high percentage of a solvent (dispersion medium)contained in the coating solution.

Specific examples of the hydrophilization treatment include publiclyknown treatments such as a chemical treatment with an acid, an alkali,or the like, a corona treatment, and a plasma treatment. Among thesehydrophilization treatments, the corona treatment is more preferable.This is because the corona treatment not only allows the separator to behydrophilized in a relatively short time but also causes only a surfaceand its vicinity of the separator to be hydrophilized and consequentlyprevents an inside of the separator from changing in quality.

(Porous Layer)

The porous layer is preferably a resin layer containing a resin. A resinof which the porous layer is made is preferably (i) insoluble in anelectrolyte of the nonaqueous electrolyte secondary battery and (ii)eletrochemically stable in a range of use of the nonaqueous electrolytesecondary battery. In a case where the porous layer is laminated to onesurface of the separator which is used as a member of the nonaqueouselectrolyte secondary battery, the porous layer is preferably laminatedto a surface of the separator which surface faces a cathode of thenonaqueous electrolyte secondary battery, and is more preferablylaminated to a surface of the separator which surface is in contact withthe cathode.

Examples of the resin of which the porous layer is made include:polyolefins such as polyethylene, polypropylene, polybutene, and anethylene-propylene copolymer; fluorine-containing resins such aspolyvinylidene fluoride (PVDP) and polytetrafluoroethylene;fluorine-containing rubbers such as a vinylidenefluoride-hexafluoropropylene copolymer, atetrafluoroethylene-hexafluoropropylene copolymer, atetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, a vinylidenefluoride-tetrafluoroethylene copolymer, a vinylidenefluoride-trifluoroethylene copolymer, a vinylidenefluoride-trichloroethylene copolymer, a vinylidene fluoride-vinylfluoride copolymer, a vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymer, and anethylene-tetrafluoroethylene copolymer; aromatic polyamide; whollyaromatic polyamide (aramid resin); rubbers such as a styrene-butadienecopolymer and a hydride thereof, a methacrylate ester copolymer; anacrylonitrile-acrylie ester copolymer, a styrene-acrylic estercopolymer, ethylene propylene rubber, and polyvinyl acetate; resinshaving a melting point or a glass transition temperature of not lessthan 180° C., such as polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamide-imide,polyether amide, and polyester; water-soluble polymers such as polyvinylalcohol, polyethylene glycol, cellulose ether, sodium alginate,polyacrylic acid, polyacrylamide, and polymethacrylic acid; and thelike.

Specific examples of the aromatic polyamide include poly(paraphenyleneterephthalamide), poly(methaphenylene isophthalamide),poly(parabenzamide), poly(methabenzamide), poly(4,4′-benzanilideterephthalamide), poly(paraphenylene-4,4′-biphenylene dicarboxylic acidamide), poly(methaphenylene-4,4′-biphenylene dicarboxylic acid amide),poly(paraphenylene-2,6-naphthalene dicarboxylic acid amide), poly(methaphyenylene-2,6-napthalene dicarboxylic acid amide), poly(2-chloroparaphenylene terephthalamide), paraphenyleneterephthalamide/2,6-dichloroparphenylene terephthalamide copolymer, andmethaphenylene terephthalamide/2,6-dichloroparaphenylene terephthalamidecopolymer. Among these aromatic polyamides, poly(paraphenyleneterephthalamide) is more preferable.

Among the above resins, polyolefin, a fluorine-containing resin,aromatic polyamide, and a hydrosoluble polymer are more preferable.Above all, a fluorine-containing resin is particularly preferable in acase where the porous layer is provided so as to face the cathode of thenonaqueous electrolyte secondary battery. Even in a ease where adeterioration in acidity occurs while the nonaqueous electrolytesecondary battery is being operated, using a fluorine-containing resinmakes it easier to maintain various performance capabilities such as arate characteristic and a resistance characteristic (solutionresistance) of the nonaqueous electrolyte secondary battery. From theviewpoint of a process and an environmental load, a hydrosolublepolymer, which allows water to be used as a solvent for forming theporous layer, is more preferable, cellulose ether and sodium alginateare still more preferable, and cellulose ether is particularlypreferable.

Specific examples of the cellulose ether include carboxymethyl cellulose(CMC), hydroxyethyl cellulose (HEC), carboxy ethyl cellulose, methylcellulose, ethyl cellulose, cyan ethyl cellulose, oxyethyl cellulose,and the like. Among these cellulose ethers, CMC and HEC, each of whichless deteriorates while being used for a long time and is excellent inchemical stability, are more preferable, and CMC is particularlypreferable.

The porous layer more preferably contains a filler. Thus, in a casewhere the porous layer contains a filler, the resin functions also as abinder resin. The filler, which is not particularly limited to anyspecific filler, can be a filler made of an organic matter or a fillermade of an inorganic matter.

Specific examples of the filler made of an organic; matter includefillers made of (i) a homopolymer of a monomer such as styrene, vinylketone, acrylonitrile, methyl methacrylate, ethyl methacrylate, glycidylmethacrylate, glycidyl acrylate, or methyl acrylate, or (ii) a copolymerof two or more of such monomers; fluorine-containing resins such aspolytetrafluoroethylene, an ethylene tetrafluoride-propylenehexafluoride copolymer, a tetrafluoroethylene-ethylene copolymer, andpolyvinylidene. fluoride; melamine resin; urea resin; polyethylene;polypropylene; polyacrylic acid and polymethacrylie acid; and the like.

Specific examples of the filler made of an inorganic matter includefillers made of Inorganic matters such as calcium carbonate, talc, clay,kaolin, silica, hydrotalcite, diatomaceous earth, magnesium carbonate,barium carbonate, calcium sulfate, magnesium sulfate, barium sulfate,aluminum hydroxide, boehmite, magnesium hydroxide, calcium oxide,magnesium oxide, titanium oxide, titanium nitride, alumina (aluminumoxide), aluminum nitride, mica, zeolite, and glass. The porous layer cancontain (i) only one kind of filler or (ii) two or more kinds of fillersin combination.

Among the above fillers, a filler made of an inorganic; matter issuitable. A filler made of an inorganic oxide such as silica, calciumoxide, magnesium oxide, titanium oxide, alumina, mica, zeolite, aluminumhydroxide, or boehmite is preferable. A filler made of at least one kindselected from the group consisting of silica, magnesium oxide, titaniumoxide, aluminum hydroxide, boehmite, and alumina is more preferable. Afiller made of alumina is particularly preferable. Alumina has manycrystal forms such as α-alumina, β-alumina, γ-alumina, and θ-alumina,and any of the crystal forms can be suitably used. Among the abovecrystal forms, α-alumina, which is particularly high in thermalstability and chemical stability, is the most preferable.

The filler has a shape that varies depending on, for example, (i) amethod for producing the organic matter or inorganic matter as a rawmaterial and (ii) a condition under which the filler is dispersed duringpreparation of a coating solution for forming the porous layer. Thefiller can have any of various shapes such as a spherical shape, anoblong shape, a rectangular shape, a gourd shape, and an indefiniteirregular shape.

In a case where the porous layer contains a filler, the filler iscontained in an amount preferably of 1% by volume to 99% by volume andmore preferably of 5% by volume to 95% by volume of the porous layer.The filler which is contained in the filler in an amount falling withinthe above range make it less likely for a void formed by a contact amongfillers to be blocked by, for example, a resin. This makes it possibleto obtain sufficient ion permeability and to set a mass per unit area ofthe porous layer at an appropriate value.

According to an embodiment of the present invention, a coating solutionfor forming the porous layer is normally prepared by dissolving theresin in a solvent and dispersing the filler in a resultant solution.

The solvent (dispersion medium), which is not particularly limited toany specific solvent, only needs to (i) have no harmful influence on theporous film, (ii) uniformly and stably dissolve the resin, and (iii)uniformly and stably disperse the filler. Specific examples of thesolvent (dispersion medium) include: water; lower alcohols such asmethyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, andt-butyl alcohol; acetone, toluene, xylene, hexane, N-methylpyrrolidone,N,N-dimethylacetamide, and N,N-dimethylformamide; and the like. Theabove solvents (dispersion media) can be used in only one kind or incombination of two or more kinds.

The coating solution can be formed by any method provided that thecoating solution can meet conditions such as a resin solid content(resin concentration) and a filler amount each necessary for obtainmentof a desired porous layer. Specific examples of a method for forming thecoating solution include a mechanical stirring method, an ultrasonicdispersion method, a high-pressure dispersion method, a media dispersionmethod, and the like.

Further, the filler can be dispersed in the solvent (dispersion medium)by use of, for example, a conventionally publicly known disperser suchas a three-one motor, a homogenizer, a media disperser, or a pressuredisperser.

In addition, the coating solution can contain, as a component differentfrom the resin and the filler, additive(s) such as a disperser, aplasticizer, a surfactant, and/or a pH adjuster, provided that theadditive(s) does/do not impair the object of the present invention. Notethat the additive(s) can be contained in an amount, that does not impairthe object, of the present invention.

A method for applying the coating solution to the separator, i.e., amethod for forming the porous layer on a surface of the separator whichhas been appropriately subjected to a hydrophilization treatment is notparticularly restricted. In a case where the porous layer is laminatedto both sides of the separator, (i) a sequential lamination method inwhich the porous layer is formed on one side of the separator and thenthe porous layer is formed on the other side of the separator, or (ii) asimultaneous lamination method in which the porous layer is formedsimultaneously on both sides of the separator is applicable to the case.

Examples of a method for forming the porous layer include: a method inwhich the coating solution is directly applied to the surface of theseparator and then the solvent (dispersion medium) is removed; a methodin which the coating solution is applied to an appropriate support, theporous layer is formed by removing the solvent, (dispersion medium), andthereafter the porous layer thus formed and the separator arepressure-bonded and subsequently the support is peeled off; a method inwhich the coating solution is applied to the appropriate support andthen the separator is pressure-bonded to an application surface, andsubsequently the support is peeled off and then the solvent (dispersionmedium) is removed; a method in which the separator is immersed in thecoating solution so as to be subjected to dip coating, and thereafterthe solvent (dispersion medium) is removed; and the like.

The porous layer can have a thickness that is controlled by adjusting,for example, a thickness of a coated film that is moist (wet) afterbeing coated, a weight ratio between the resin and the filler, and/or asolid content concentration (a sum of a resin concentration and a fillerconcentration) of the coating solution. Note that it is possible to use,as the support, a film made of resin, a belt made of metal, or a drum,for example.

A method for applying the coating solution to the separator or thesupport is not particularly limited to any specific method provided thatthe method achieves a necessary mass per unit area and a necessarycoating area. It is possible to employ a conventionally publicly knownmethod as a coating solution application method. Specific examples ofsuch a conventionally publicly known coating solution application methodinclude a gravure (c)outer method, a small-diameter gravure coatermethod, a reverse roll coater method, a transfer roll coater method, akiss coater method, a dip coater method, a knife coater method, an airdoctor blade coater method, a blade coater method, a rod coater method,a squeeze coater method, a cast coater method, a bar coater method, adie coater method, a screen printing method, a spray application method,and the like.

Generally, the solvent (dispersion medium) is removed by drying.Examples of a drying method include natural drying, air-blowing drying,heat drying, vacuum drying, and the like. Note, however, that any dryingmethod is usable provided that the drying method allows the solvent(dispersion medium) to be sufficiently removed. For the drying, it ispossible to use an ordinary drying device.

Further, it is possible to carry out the drying after replacing thesolvent (dispersion medium) contained in the coating solution withanother solvent. Examples of a method for removing the solvent(dispersion medium) after replacing the solvent (dispersion medium) withanother solvent include a method, in which a resin is dissolved in. thesolvent (dispersion medium) contained in the coating solution, anothersolvent (hereinafter referred to as a solvent X) that does not dissolvethe resin contained in coating solution is used, the separator or thesupport on which a, coated film has been formed by application of thecoating solution is immersed in the solvent X, the solvent (dispersionmedium) contained in the coated film formed on the separator or thesupport is replaced with the solvent X, and thereafter the solvent X isevaporated. This method makes it possible to efficiently remove thesolvent (dispersion medium) from the coating solution.

Assume that heating is carried out so as to remove the solvent.(dispersion medium) or the solvent X from the coated film of the coatingsolution which coated film has been formed on the separator or thesupport. In this case, in order to prevent the separator from having alower air permeability due to contraction of pores of the separator, itis desirable to carry out heating at a temperature at which theseparator does not have a lower air permeability, specifically, 10° C.to 120° C., more preferably 20C to 80° C.

In a case where the separator is used as the base material to form thelaminated separator by laminating the porous layer to one side or bothsides of the separator, the porous layer formed by the method describedearlier has, per one side thereof, a film thickness preferably of 0.5 μmto 15 μm and more preferably of 2 μm to 10 μm.

The porous layer whose both sides have a film thickness of less than 1μm in total cannot sufficiently prevent an internal short circuit causedby, for example, breakage in a nonaqueous electrolyte secondary batterywhich includes the laminated separator. Furthermore, such a porous layerretains a smaller amount of electrolyte.

Meanwhile, the porous layer whose both sides have a film thickness ofmore than 30 μm in total causes an increase in permeation resistance oflithium ions in the entire laminated separator which is included in anonaqueous electrolyte secondary battery. Thus, in a case where chargeand discharge cycles are repeated, a cathode of the nonaqueouselectrolyte secondary battery deteriorates and consequently decreases inrate characteristic and/or cycle characteristic. Furthermore, such aporous layer increases a distance between the cathode and an anode ofthe nonaqueous electrolyte secondary battery. This makes the nonaqueouselectrolyte secondary battery larger in size.

In a case where the porous layer is laminated to both sides of theseparator, physical properties of the porous layer which are describedbelow at least refer to physical properties of the porous layer which islaminated to a surface of the laminated separator which surface facesthe cathode of the nonaqueous electrolyte secondary battery whichincludes the laminated separator.

The porous layer only needs to have, per one side thereof, a mass perunit area which mass is appropriately determined in view of a strength,a film thickness, a weight, and handleability of the laminatedseparator. In a case where the nonaqueous electrolyte secondary batteryincludes the laminated separator, the porous layer normally has a massper unit area preferably of 1 g/m² to 20 g/m² and more preferably of 2g/m² to 10 g/m².

The porous layer which has a mass per unit area which mass falls withinthe above range allows the nonaqueous electrolyte secondary batteryincluding the porous layer to have a higher weight energy density and ahigher volume energy density. Meanwhile, the porous layer which has amass per unit area which mass is beyond the above range causes thenonaqueous electrolyte secondary battery including the laminatedseparator to have a greater weight.

The porous layer has a porosity preferably of 20% by volume to 90% byvolume and more preferably of 30% by volume to 8 0% by volume so thatsufficient ion permeability can be obtained. Further, the porous layerhas pores having a pore size preferably of not more than 1.0 μm and morepreferably of not more than 0.5 μm. The porous layer which has poreshaving a pore size falling within the above range allows the nonaqueouselectrolyte secondary battery which includes the laminated separatorincluding such a porous layer to obtain sufficient ion permeability.

The laminated separator has a Gurley air permeability preferably of 30sec/100 mL to 1000 sec/100mL and more preferably of 50 sec/100 mL to 800sec/100mL. The laminated separator which has a Gurley air permeabilityfalling within the above range makes it possible to obtain sufficiention permeability in a case where the laminated separator is used as amember for the nonaqueous electrolyte secondary battery.

Meanwhile, the laminated separator which has a Gurley air permeabilitybeyond the above range means that the laminated separator has a coarselaminated structure due to a high porosity thereof. This causes thelaminated separator to have a lower strength, so that the laminatedseparator may be insufficient in shape stability, particularly shapestability at a high temperature. In contrast, the laminated separatorwhich has a Gurley air permeability falling below the above range makesit impossible to obtain sufficient ion permeability in a case where theseparator is used as a member for the nonaqueous electrolyte secondarybattery. This may cause the nonaqueous electrolyte secondary battery tohave a lower battery characteristic.

As described earlier, the separator included in the laminated separatorin accordance with an embodiment of the present invention has ΔWI of notmore than 2.5 and thus contains the oxide in a smaller amount than aconventionally known separator. Thus, as in the case of the separator,the laminated separator allows a side reaction to be less likely tooccur during charge and discharge of a nonaqueous electrolyte secondarybattery, and consequently makes it possible to provide a nonaqueouselectrolyte secondary battery that has an excellent cyclecharacteristic.

<Nonaqueous Electrolyte Secondary Battery Member, Nonaqueous ElectrolyteSecondary Battery>

The nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention includes the separator (describedearlier) or the laminated separator (described earlier) (hereinaftereach of the separator and the laminated separator may also becollectively referred to as a “separator or the like”). Morespecifically, the nonaqueous electrolyte secondary battery in accordancewith an embodiment of the present invention includes a nonaqueouselectrolyte secondary battery member including a cathode, a separator orthe like, and an anode that are provided in this order. That is, thenonaqueous electrolyte secondary battery member is also encompassed inthe scope of the present invention. The following description takes alithium ion secondary battery member as an example of the nonaqueouselectrolyte secondary battery. Note that components of the nonaqueouselectrolyte secondary battery except the separator are not limited tothose discussed in the following description.

In the nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention., it is possible to use, forexample, a nonaqueous electrolyte obtained by dissolving lithium salt inan organic solvent, Examples of the lithium salt include LiClO₄, LiPF₆,LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiC(CF₃S₂)₃, Li₂B₁₀Cl₁₀,lower aliphatic carboxylic acid lithium salt, LiAlCl₄, and the like. Theabove lithium salts can be used in only one kind or in combination oftwo or more kinds.

Of the above lithium salts, at least one kind of fluorine-containinglithium sail selected from the group consisting of LiPF₆, LiAsF₆,LiSbF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, and LiC(CF₃SO₂)₃ is morepreferable.

Specific examples of the organic solvent of the nonaqueous electrolyteinclude: carbonates such as ethylene carbonate, propylene carbonate,dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate,4-trifluoromethyl-1,3-dioxolane-2-one, and1,2-di(methoxycarbonyloxy)ethane; ethers such as 1,2-dimethoxyethane,1,3-dimethoxypropane, pentafluoropropylmethyl ether,2,2,3,3-tetrafluoropropyldifluoromethylether, tetrahydrofuran, and2-methyltetrahydrofuran; esters such as methyl formate, methyl acetate,and γ-butyrolactone; nitrites such as acetonitrile and butyronitrile;amides such as N,N-dimethylformamide and N,N-dimethylacetamidecarbamates such as 3-methyl-2-oxazolidone; sulfur-containing compoundssuch as sulfolane, dimethylsulfoxide, and 1,3-propanesultone; afluorine-containing organic solvent obtained by introducing a fluorinegroup in the organic solvent; and the like. The above organic solventscan be used in only one kind or in combination of two or more kinds.

Of the above organic solvents, a carbonate is more preferable, and amixed solvent of cyclic carbonate and acyclic carbonate or a mixedsolvent of cyclic carbonate and an ether is more preferable.

The mixed solvent of cyclic carbonate and acyclic carbonate is morepreferably exemplified by a mixed solvent containing ethylene carbonate,dimethyl carbonate, and ethyl methyl carbonate. This is because themixed solvent containing ethylene carbonate, dimethyl carbonate, andethyl methyl carbonate operates in a wide temperature range, and isrefractory also in a case where a graphite material such as naturalgraphite or artificial graphite is used as an anode active material.

Normally, a sheet cathode in which a cathode current, collector supportsthereon a cathode mix containing a cathode active material, anelectrically conductive material, and a binding agent is used as thecathode.

Examples of the cathode active material include a material that iscapable of doping and dedoping lithium ions. Examples of such a materialinclude lithium complex oxides each containing at least one kind oftransition metal selected from the group consisting of V, Mn, Fe, Co,and Ni.

Of the above lithium complex oxides, a lithium complex oxide having anα-NaFeO₂ structure, such as lithium nickel oxide or lithium cobaltoxide, or a lithium complex oxide having a spinel structure, such aslithium manganate spinel is more preferable. This is because such alithium complex oxide is high in average discharge potential. Thelithium complex oxide can contain various metallic elements, and lithiumnickel complex oxide is more preferable.

Further, it is particularly preferable to use lithium, nickel complexoxide which contains at least one kind of metallic element so that theat least one kind of metallic element accounts for 0.1 mol % to 20 mol %of a sum of the number of moles of the at least one kind of metallic,element and the number of moles of Ni in lithium nickel oxide, the atleast one kind of metallic element being selected from the groupconsisting of Ti, Zr, Ce, Y, V, Cr, Mn, Fe, Co, Cu, Ag, Mg, Al, Ga, In,and Sn. This is because such lithium nickel complex oxide is excellentin cycle characteristic during use of the nonaqueous electrolytesecondary battery at a high capacity. Especially an active materialwhich contains Al or Mn and has an Ni content, of not less than 85 mol %and more preferably of not less than 90 mol % is particularlypreferable. This is because such an active material is excellent incycle characteristic during use of the nonaqueous electrolyte secondarybattery at a high capacity, the nonaqueous electrolyte secondary batteryincluding the cathode containing the active material. Note here thatrelative to a sum (100%) of the number of moles (mol %) of Al or Mn andthe number of moles (mol %) of Ni in lithium nickel oxide, Al or Mn iscontained in an amount of 0.1 mol % to 20 mol %, and Ni is contained inan amount of not less than 85 mol % and more preferably of not less than90 mol %.

Examples of the electrically conductive material include carbonaceousmaterials such as natural graphite, artificial graphite, cokes, carbonblack, pyrolytic carbons, carbon fiber, organic high molecular compoundbaked bodies, and the like. The above electrically conductive materialscan be used in only one kind. Alternatively, the above electricallyconductive materials can be used in combination of two or more kinds by,for example, mixed use of artificial graphite and carbon black.

Examples of the binding agent include polyvinylidene fluoride, avinylidene fluoride copolymer, polytetrafluoroethylene, a vinylidenefluoride-hexafluoropropylene copolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, atetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, anethylene-tetrafluoroethylene copolymer, a vinylidenefluoride-tetrafluoroethylene copolymer, a vinylidenefluoride-trifluoroethylene copolymer, a vinylidenefluoride-trichloroethylene copolymer, and a vinylidene fluoride-vinylfluoridecopolymer, a vinylidenefluoride-hexafluoropropylene-tetrafluoroethlene copolymer, thermoplasticresins such as thermoplastic polyimide, thermoplastic polyethylene, andthermoplastic polypropylene, acrylic resin, and styrene butadienerubber. Note that the binding agent also functions as a thickener.

The cathode mix can be obtained by, for example, pressing the cathodeactive material, the electrically conductive material, and the bindingagent on the cathode current collector, or causing the cathode activematerial, the electrically conductive material, and the binding agent tobe in a form of paste by use of an appropriate organic solvent.

Examples of the cathode current collector include electricallyconductive materials such as Al, Ni, and stainless steel, and Al, whichis easy to process into a thin film and less expensive, is morepreferable.

Examples of a method for producing the sheet cathode, i.e., a method forcausing the cathode current collector to support the cathode mixinclude: a method in which the cathode active material, the electricallyconductive material, and the binding agent which are to be formed intothe cathode mix are pressure-molded on the cathode current collector; amethod in which the cathode current collector is coated with the cathodemix which has been obtained by causing the cathode active material, theelectrically conductive material, and the binding agent to be in a formof paste by use of an appropriate organic solvent, and a sheet cathodemix obtained, by drying is pressed so as to be closely fixed to thecathode current, collector; and the like.

Normally, a sheet anode in which an anode current collector supportsthereon an anode mix containing an, anode active material is used as theanode. The sheet anode preferably contains the electrically conductivematerial and the binding agent.

Examples of the anode active material include a material that is capableof doping and dedoping lithium ions, lithium metal or lithium alloy, andthe like. Specific examples of such a material include; carbonaceousmaterials such as natural graphite, artificial graphite, cokes, carbonblack, pyrolytic carbons, carbon fiber, and organic high molecularcompound baked bodies; chalcogen compounds such as oxides and sulfideseach doping and dedoping lithium ions at a lower potential than that ofthe cathode; metals such as aluminum (Al), lead (Pb), tin (Sn), bismuth(Bi), and silicon (Si) each alloyed with an alkali metal; cubicintermetallic compounds (AlSb, Mg₂Si, NiSi₂) having lattice spaces inwhich alkali metals can be provided; lithium nitrogen compounds(Li_(3-x)M_(x)N (M: transition metal)); and the like.

Of the above anode active materials, a carbonaceous material whichcontains, as a main component, a graphite material such as naturalgraphite or artificial graphite is preferable. This is because such acarbonaceous material is high in potential evenness, and a great energydensity can be obtained in a case where the carbonaceous material, whichis low in average discharge potential, is combined with the cathode. Ananode active material which is a mixture of a graphite material andsilicon and has an Si to C ratio of not less than 5% is more preferable,and an anode active material which is a mixture of a graphite materialand silicon and has an Si to C ratio of not less than 10% is still morepreferable. That is, Si is preferably contained in an amount of not lessthan 5 mol % and more preferably of 10 mol % relative to a sum (100 mol%) of the number of moles of C, which is the graphite material, and thenumber of moles of Si.

The anode mix can be obtained by, for example, pressing the anode activematerial on the anode current collector, or causing the anode activematerial to be in a form of paste by use of an appropriate organicsolvent.

Examples of the anode current collector include Cu, Mi, stainless steel,and the like, and Cu, which is difficult, to alloy with lithiumparticularly in a lithium ion secondary battery and easy to process intoa thin film, is more preferable.

Examples of a method for producing the sheet anode, i.e., a method forcausing the anode current collector to support the anode mix include: amethod in which the anode active material to be formed into the anodemix are pressure-molded on the anode current collector; a method inwhich the cathode current, collector is coated with the anode mix whichhas been obtained by causing the anode active material to be in a formof paste by use of an appropriate organic solvent, and a sheet anode mixobtained by drying is pressed so as to be closely fixed to the anodecurrent collector; and the like. The paste preferably contains theelectrically conductive material and the binding agent.

The nonaqueous electrolyte secondary battery member in accordance withan embodiment of the present invention is formed by providing thecathode, the separator or the like, and the anode in this order.Thereafter, the nonaqueous electrolyte secondary battery member isplaced in a container serving as a housing of the nonaqueous electrolytesecondary battery. Subsequently, the container is filled with anonaqueous electrolyte, and then the container is sealed while beingdecompressed. The nonaqueous electrolyte secondary battery in accordancewith an embodiment of the present invention, can thus be produced. Thenonaqueous electrolyte secondary battery, which is not particularlylimited in shape, can have any shape such as a sheet, (paper) shape, adisc shape, a cylindrical shape, or a prismatic shape such as arectangular prismatic shape. Note that, a method for producing thenonaqueous electrolyte secondary battery is not particularly limited toany specific method, and a conventionally publicly known productionmethod can be employed as the method.

As described earlier, the nonaqueous electrolyte secondary battery inaccordance with an embodiment of the present invention includes (i) aseparator having ΔWI of not more than 2.5 or (ii) a laminated separatorincluding the separator and a porous layer. Such a separator contains asmaller amount of an oxide of a resin which oxide may cause a sidereaction during charge and discharge of a battery. This allows thenonaqueous electrolyte secondary battery to have an excellent cyclecharacteristic. For example, Examples (later later) show a highdischarge capacity maintaining ratio of more than 70.0%.

The discharge capacity maintaining ratio is a ratio of (a) a dischargecapacity obtained after a new nonaqueous electrolyte secondary batterythat has not been subjected to a charge discharge cycle is subjected togiven cycles of charge and discharge to (b) a discharge capacityobtained after that nonaqueous electrolyte secondary battery has beensubjected to the first cycle of the charge and discharge. A higherdischarge capacity maintaining ratio means that a battery is moreexcellent in cycle characteristic, i.e., a battery has a longer life.How to calculate the discharge capacity maintaining ratio will bedescribed later in Examples.

A battery that has a discharge capacity maintaining ratio, calculatedbased on Equation (3) (described later in Examples), of not less than70.0% can be said to have a sufficient cycle characteristic from theviewpoint of battery life extension.

The present invention is not limited to the embodiments, but can bealtered by a skilled person in the art within the scope of the claims.An embodiment derived from a proper combination of technical means eachdisclosed in a different embodiment is also encompassed in the technicalscope of the present invention.

EXAMPLES

The following description will more specifically discuss the presentinvention with reference to Examples and Comparative Example. Note,however, that the present invention is not limited to such Examples andComparative Examples.

<Method for Measuring Physical Properties etc.>

Physical properties etc. of separators and porous layers of Examples andComparative Example were measured as below,

(1) Film Thickness (Unit: μm)

A film thickness was measured by use of a high-accuracy digital lengthmeasuring machine manufactured by Mitsutoyo Corporation.

(2) Porosity (Unit: %):

A sample in a form of an eight-centimeter square was cut out from thefilm, and a weight W (g) and a thickness D (cm) of the sample weremeasured, A volume of each material contained in the sample wascalculated by calculating a weight of the each material and dividing aweight Wi (g) of the each, material, by an absolute specific gravity,and then a porosity is calculated based on the following equation:

Porosity (% by volume)=100−[{(W1/absolute specific gravity1)+(W2/absolute specific gravity 2)+ . . . +(Wn/absolute specificgravity n)}/(8×8×D)]×100

(3) Weatherability Rest

In conformity with JIS B 7753, test pieces obtained from porous films(separators) produced in Production Examples and Comparative ProductionExamples were irradiated with ultraviolet light by use of a sunshineweather meter S80 (manufactured by Suga Test Instruments Co., Ltd).Specifically, each of the test pieces was irradiated with ultravioletlight having an intensity of 255 W/m² for 75 hours by use of a sunshinecarbon arc light lamp (four pairs of ultra long life carbon), which wasset to have a discharge voltage of 50 V and a discharge current of 60 A,and under conditions of a black panel temperature of 60° C. and arelative humidity of 50%.

(4) Measurement of White Index (WI)

WI of the separator was measured by Specular Component included (SCI)method (including specular reflection) by use of a spectrocolorimeter(CM-2002, manufactured, by MINOLTA.). During the measurement of WI, theseparator was placed on black paper (manufactured by Hokuetsu KisliuPaper Co., Ltd. , colored high- quality paper, black, thickest type,shirokuhan (788 mm×1091 mm with a long side extending in a machinedirection)). Then, ΔWI was calculated based on Equation (1) (describedearlier) where WI₀ is WI measured before the separator was subjected tothe weatherability test, and WI₁ is WI measured after the separator wassubjected to the weatherability test.

(5) Discharge Capacity Maintaining Ratio

A new nonaqueous electrolyte secondary battery, which had not beensubjected to a charge and discharge cycle, was subjected to three cyclesof initial charge and discharge. Each of the three cycles of the initialcharge and discharge was carried out at 25° C., at a voltage rangingfrom 4,1 V to 2.7 V, and at an electric current value of 0.2 C. Notethat a value of an electric current at which a battery rated capacitydefined as a one-hour rate discharge capacity is discharged in one houris assumed to be 1 C. This applies also to the following descriptions.Subsequently, three cycles of charge and discharge were carried out ateach of electric current values 1 C, 5 C, 10 C, and 20 C. Lastly, threecycles of charge and discharge were carried out at an electric currentvalue of 0.2 C, and then a discharge capacity maintaining ratio after 18cycles of charge and discharge was calculated based on the followingEquation (2):

Discharge capacity maintaining ratio after 18 cycles (%)=(dischargecapacity obtained in the 18th cycle at 0.2 C/discharge capacity obtainedin the first cycle at 0.2 C)×100  (2)

A discharge capacity maintaining ratio after 180 cycles of charge anddischarge was further calculated based on the following Equation (3):

Discharge capacity maintaining ratio after 180 cycles (%)=(dischargecapacity obtained in the 18th cycle at 0.2C/discharge capacity obtainedin the first cycle at 0.2 C)¹⁰×100  (3)

PRODUCTION EXAMPLES

<Production of Separators>

Production Example 1

A polyethylene powder (ultra-high molecular weight polyethylene GUR2024manufactured by Ticona Corporation) and a low-molecular-weightpolyethylene powder (polyethylene wax FNP-01.15 manufactured by NipponSeiro Co., Ltd,) having a weight-average molecular weight of 1,000 weremixed so as to obtain a resin mixture in which the polyethylene powderand the low-molecular-weight polyethylene powder were contained inrespective amounts of 68% by weight and 32% by weight. Then, with 100parts by weight, of the resin mixture, 160 parts by weight of calciumcarbonate (manufactured by Maruo Calcium Co., Ltd, and having an averageparticle size of 0.10 μm) and 3 parts by weight of an antioxidant(IRG1010 (manufactured by Ciba Specialty Chemicals Corporation):Irf168(manufactured by Ciba Specialty Chemicals Corporation)=2 parts byweight:1 part by weight) were mixed. A resultant mixture wasmelt-kneaded by use of a twin screw kneading extruder at 200° C., sothat a resin composition was obtained.

The resin, composition was extruded from a T-die, which had been set at240° C., so as to be sheet-shaped. Then, the resin composition wasrolled by a pair of rollers having a surface temperature of 150° C., sothat, a sheet of the resin composition was prepared. An air exposuretime in which the resin composition was extruded from the T-die and thencooled by the rollers (i.e., a period in which the resin composition wasextruded from the T-die and then, brought into contact with the rollers)was 3.6 seconds. Note that in all Production Examples and ComparativeProduction Examples, an identical extrusion machine was used and adistance (corresponding to the distance 4 illustrated in FIG. 1) betweenthe T-die and the rollers was 15 cm.

The sheet thus prepared was immersed in an aqueous hydrochloric acidsolution (containing 4 mol/L of hydrochloric acid and 1% by weight of anon-ionic surfactant (SANMORIN 11 manufactured by Sanyo Chemicalindustries, Ltd.)) at 4° C., so that calcium carbonate was removed.Then, the sheet was stretched sixfold at 100° C. by use of a tenteruniaxial stretching machine manufactured by Ichikin Co., Ltd, so that aseparator 1, which is a polyolefin porous film, was obtained.

Production Example 2

A polyethylene powder (ultra-high molecular weight polyethylene GUR4032manufactured by Ticona Corporation) and a low-molecular-weightpolyethylene powder (polyethylene wax FNP-0115 manufactured by NipponSeiro Co,, Ltd.) were mixed so as to obtain a resin mixture in which thepolyethylene powder and the low-molecular-weight polyethylene powderwere contained in respective amounts of 70% by weight and 30% by weight.Then, with 100 parts by weight of the resin mixture. 160 parts by weightof calcium carbonate (manufactured by Maruo Calcium Co., Ltd, and havingan average particle size of 0.10 μm) and 3 parts by weight of anantioxidant (IRG1010:Irf168=2 parts by weight:1 part by weight) weremixed. A resultant mixture was melt-kneaded by use of a twin screwkneading extruder at 200° C., so that a resin composition was obtained.

The resin composition was extruded from a T-die. which had been set at247° C., so as to be sheet-shaped, Then, the resin composition wasrolled by a pair of rollers having a surface temperature of 150° C., sothat a sheet of the resin composition was prepared. An air exposure timein which the resin composition was extruded from the T-die and thencooled by the rollers was 3.0 seconds.

The sheet thus prepared was immersed in an aqueous hydrochloric acidsolution (containing 4 mol/L of hydrochloric acid and 6% by weight of anon-ionic surfactant) at 40° C., so that calcium carbonate was removed.Then, the sheet was stretched sixfold at 105° C. by use of a tenteruniaxial stretching machine manufactured by Ichikin Co., Ltd, so that aseparator 2, which is a polyolefin porous film, was obtained,

Comparative Production Example 1

A polyethylene powder (ultra-high molecular weight polyethylene GUR4032manufactured by Ticona Corporation) and a low-molecular-weightpolyethylene powder (polyethylene wax FNP-0115 manufactured by NipponSeiro Co., Ltd.) were mixed so as to obtain a resin mixture in which thepolyethylene powder and the low-molecular-weight polyethylene powderwere contained in respective amounts of 71% by weight and 29% by weight.Then, with 100 parts by weight of the resin mixture, 160 parts by weightof calcium carbonate (manufactured by Ma mo Calcium Co., Ltd, and havingan average particle size of 0.10 μm) and 3 parts by weight of anantioxidant (IRG1010:Irf168=2 parts by weight:1 part by weight) weremixed. A resultant mixture was melt-kneaded by use of a twin screwkneading extruder at 200° C., so that a resin composition was obtained.

The resin composition was extruded from a T-die, which had been set at253° C., so as to be sheet-shaped. Then, the resin composition wasrolled by a pair of rollers having a surface temperature of 150° C., sothat a sheet of the resin composition was prepared. An air exposure timein which, the resin composition was extruded from the T-die and thencooled by the rollers was 2.3 seconds.

The sheet thus prepared was immersed in an aqueous hydrochloric acidsolution (containing 4 mol/L of hydrochloric acid and 1% by weight ofnon-ionic surfactant) at 40° C., so that calcium carbonate was removed.Then, the sheet was stretched sevenfold at 100° C. by use of a tenteruniaxial stretching machine manufactured by Ichikin Co., Ltd. so that aseparator 1 for comparison (hereinafter referred to as a “comparativeseparator 1”), which is a polyolefin porous film, was obtained.

Comparative Production Example 2

A polyethylene powder (ultra-high molecular weight polyethylene GUR4032manufactured by Ticona Corporation) and a low-molecular-weightpolyethylene powder (polyethylene wax FNP-0115 manufactured by NipponSeiro Co., Ltd.) were mixed so as to obtain a resin mixture in which thepolyethylene powder and the low-molecular-weight polyethylene powderwere contained in respective amounts of 70% by weight and 30% by weight.Then, with 100 parts by weight of the resin mixture, 160 parts by weightof calcium carbonate (manufactured by Maruo Calcium Co., Ltd. and havingan average particle size of 0.10 μm) and 3 parts by weight of anantioxidant (IRG1010:Irf168=2 parts by weight:1 part, by weight) weremixed. A resultant mixture was melt-kneaded by use of a twin screwkneading extruder at 200° C., so that a resin composition was obtained.

The resin composition was extruded from, a T-die, which had been set at252° C., so as to be sheet-shaped. Then, the resin composition wasrolled by a pair of rollers having a surface temperature of 150° C., sothat a sheet of the resin composition was prepared. An air exposure timein which the resin composition was extruded from the T-die and thencooled by the rollers was 3.6 seconds.

The sheet thus prepared was immersed in an aqueous hydrochloric acidsolution (containing 4 mol/L of hydrochloric acid and 6% fby weight ofnon-ionic surfactant) at 40° C., so that calcium carbonate was removed.Then, the sheet was stretched sixfold at 105° C. by use of a tenteruniaxial stretching machine manufactured by Ichikin Co., Ltd, so that aseparator 2 for comparison (hereinafter referred to as a “comparativeseparator 2”), which, is a polyolefin porous film, was obtained.

Preparation of Nonaqueous Electrolyte Secondary Battery>

Next, nonaqueous electrolyte secondary batteries were produced by thefollowing method by use of the separators 1 and 2, which were preparedin respective Production Examples, and the comparative separators 1and2, which were prepared in respective Comparative Production Examples.

(Cathode)

A commercially-available cathode produced by applying, to aluminum foil,a mixture of 92 parts by weight of LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂, whichis a cathode active material, 5 parts by weight of an electricallyconductive material, and 3 parts by weight of polyvinylidene fluoride(PVDF) was used to prepare a nonaqueous electrolyte secondary battery.The aluminum foil was cut out so that a first part of the aluminum foilin which first part no cathode active material layer was provided andwhich first part had a width of 13 mm was left around a second part ofthe aluminum foil in which second part a cathode active material layerwas provided and which second part had a size of 40 mm×35 mm. A cathodewas thus obtained. The cathode active material layer had a thickness of58 μm and a density of 2.50 g/cm³.

(Anode)

A commercially-available anode produced by applying, to a copper foil, amixture of 93 parts by weight of graphite, which is an anode activematerial, 1 parts by weight of a styrene-1,3-butadiene copolymer, and 1parts by weigh of carboxymethyl cellulose sodium was used to prepare anonaqueous electrolyte secondary battery. The copper foil of the anodewas cut so that a first part of the copper foil in which first part noanode active material layer was provided and which first part had awidth of 13 mm was left around a second part of the copper foil in whichsecond part an anode active material layer was provided and which secondpart had a size of 50 mm×40 mm. An anode was thus obtained. The anodeactive material layer had a thickness of 49 μm and a density of 1.40g/cm³.

(Assembly)

The cathode, the separator (separator 1 or 2, or comparative separator 1or 2), and the anode were laminated (provided) in this order in alaminate pouch, so that a nonaqueous electrolyte secondary batterymember was obtained. In this case, the cathode and the anode werepositioned so that a whole of a main surface of the cathode activematerial layer of the cathode was included in a range of a main surface(overlapped the main surface) of the anode active material layer of theanode.

Subsequently, the nonaqueous electrolyte secondary battery member wasplaced in a bag obtained by laminating an aluminum layer and a heat seallayer, and 0.25 ml of a nonaqueous electrolyte was poured into the bag.The nonaqueous electrolyte was an electrolyte having a temperature of25° C. and obtained by dissolving LiPF₆ having a concentration of 1.0mole per liter in a mixed solvent of ethyl methyl carbonate, diethylcarbonate, and ethylene carbonate in a volume ratio of 50:20:30. Then,the bag was heat-sealed while being decompressed, so that nonaqueouselectrolyte secondary batteries 1 and 2, which include the respectiveseparators 1 and 2, and comparative nonaqueous electrolyte secondarybatteries 1 and 2, which, include the respective comparative separators1 and 2, were prepared.

Examples 1 and 2 and Comparative Examples 1 and 2

In Examples 1 and 2, discharge capacity maintaining ratios after 180cycles of the nonaqueous electrolyte secondary batteries 1 and 2, whichinclude the respective separators 1 and 2, were calculated. InComparative Examples 1 and 2, discharge capacity maintaining ratiosafter 180 cycles of the comparative nonaqueous electrolyte, secondarybatteries 1 and 2, which include the respective comparative separators 1and 2, were calculated. Table 1 shows results of the calculations.

Extrusion Temperature, Air Exposure Time, and Surfactant Concentration,each of which is shown in Table 1, are conditions under which to produceeach of the separators used in respective Examples and ComparativeExamples, and are described in Production Examples (described earlier)and Comparative Production Examples (described earlier), “SurfactantConcentration” shown in Table 1 refers to a concentration of a non-ionicsurfactant contained in the aqueous hydrochloric acid solution.

Film thickness, Porosity, and ΔWI, each of which is shown in. Table 1,refer to a film thickness, a porosity, and ΔWI, respectively, of each ofthe separators used In Examples and Comparative Examples.

TABLE 1 Extrusion Air Surfactant Film Discharge Capacity TemperatureExposure Concentration Thickness Porosity Maintaining Ratio (° C.) Time(Sec) (wt %) (μm) (%) ΔWI After 180 Cycles (%) Example 1 240 3.6 1 11 372.19 70.7 Example 2 247 3.0 6 16 52 2.16 70.9 Comparative 253 2.3 1 1250 3.26 69.4 Example 1 Comparative 252 3.6 6 16 65 2.79 67.8 Example 2

As shown in Table 1, it is revealed that the nonaqueous electrolyte,secondary batteries 1 and 2, which include the respective separators 1and 2 each having ΔWI of not more than 2.5, each had a dischargecapacity maintaining ratio after 180 cycles of more than 70,0%, i.e.,had a high cycle characteristic. In contrast, the comparative nonaqueouselectrolyte secondary batteries 1 and 2, which include the respectivecomparative separators 1 and 2 each having ΔWI of more than 2,5, eachhad a discharge capacity maintaining ratio of less than 70.0%. From theviewpoint of battery life extension, a nonaqueous electrolyte secondarybattery which has a discharge capacity maintaining ratio of less than70.0% can be said to have an insufficient cycle characteristic.

As shown in Table 1, the resin was extruded from the T-die at extrusiontemperatures of 240° C. and 247° C. in respective Examples 1 and 2,whereas the resin was extruded from the T-die at extrusion temperaturesof 253° C. and 252° C. in respective Comparative Examples 1 and 2.

As described earlier, since Examples 1 and 2 each used a separatorobtained from a resin that had been exposed to air at a temperaturelower than a temperature, at which a resin used in each of ComparativeExamples 1 and 2 had been exposed to air, the separators 1 and 2 eachhad ΔWI of not more than 2.5. Thus, the nonaqueous electrolyte secondarybatteries 1 and 2 are considered to have had an excellent cyclecharacteristic.

In each of Comparative Production Examples 1 and 2, the extrusiontemperature exceeded 250° C. Thus, it is considered that an oxidecontained in a resin increased in amount in a period in which the resinwas extruded and then brought into contact with the rollers, and aresultant separator had ΔWI of more than 2.5, so that the comparativenonaqueous electrolyte secondary batteries 1 and 2 each including theresultant separator had an insufficient discharge capacity maintainingratio of less than 70.0%.

Comparative Production Example 1, in which the air exposure time was 2.3seconds, was the shortest in air exposure time of all ProductionExamples and Comparative Production Examples. Note, however, that theextrusion temperature was 253° C. in Comparative Production Example 1.Thus, it is considered that an influence of a high extrusion temperatureof higher than 250° C. was greater than an advantage brought by a shortair exposure time.

As proved by Examples, it is knowledge found by the present inventionfor the first time that a nonaqueous electrolyte secondary batteryhaving a high cycle characteristic can be obtained by using a separatorhaving ΔWI of not more than 2.5. Thus, the present invention can be saidto be extremely useful as an invention that makes it possible to providea nonaqueous electrolyte secondary battery that can sufficiently meet ademand for battery life extension.

INDUSTRIAL APPLICABILITY

The present invention is widely applicable to industrial fields in eachof which a nonaqueous electrolyte secondary battery is used, e.g.,fields of a personal computer, a mobile phone, and a portableinformation terminal.

REFERENCE SIGNS LIST

1 Resin

2 T-die

3 Roller

4 Distance between T-die and roller

1. A nonaqueous electrolyte secondary battery separator comprising aporous film containing not less than 50% by volume of a polyolefin-basedresin having a weight-average molecular weight of 5×10⁵ to 15×10⁶, thenonaqueous electrolyte secondary battery separator having ΔWI of notmore than 2.5, ΔWI being defined by the following Equation (1):ΔWI=WI ₁ −WI ₀  Equation (1) where WI is a white index defined byAmerican Standard Test Method (ASTM) E313, WI₀ is WI that is measured,by use of a spectrocolorimeter, on a surface of the porous film whichhas not been irradiated with ultraviolet light having 255 W/m², and Withis WI that is measured, by use of the spectrocolorimeter, on the surfaceof the porous film which has been irradiated, for 75 hours, with theultraviolet light having 255 W/m².
 2. A nonaqueous electrolyte secondarybattery laminated separator comprising: a nonaqueous electrolytesecondary battery separator recited in claim 1; and a porous layer.
 3. Anonaqueous electrolyte secondary battery member comprising: a cathode; anonaqueous electrolyte secondary battery separator recited in claim 1;and an anode, the cathode, the nonaqueous electrolyte secondary batteryseparator, and the anode being provided in this order.
 4. A nonaqueouselectrolyte secondary battery member comprising: a cathode; a nonaqueouselectrolyte secondary battery laminated separator recited in claim 2;and an anode, the cathode, the nonaqueous electrolyte secondary batterylaminated separator, and the anode being provided in this order.
 5. Anonaqueous electrolyte secondary battery comprising: a nonaqueouselectrolyte secondary battery separator recited in claim
 1. 6. Anonaqueous electrolyte secondary battery comprising: a nonaqueouselectrolyte secondary battery laminated separator recited in claim
 2. 7.A method for producing a nonaqueous electrolyte secondary batteryseparator that comprises a porous film containing a polyolefin-basedresin as a main component, the method comprising the steps of: extrudinga resin from an outlet of an extrusion machine; and causing a pair ofrollers to roll the resin so as to form the resin into a thin film, theresin being extruded from the outlet at a temperature of not lower than200° C. and not higher than 250° C., and the resin being exposed to airfor not longer than 3.6 seconds after being extruded from the outlet andthen brought into contact with the pair of rollers, the nonaqueouselectrolyte secondary battery separator having ΔWI of not more than 2.5,ΔWI being defined by the following Equation (1):ΔWI=WI ₁ −WI ₀  Equation (1) where WI is a white index defined byAmerican Standard Test Method (ASTM) E313; WI₀ is WI that is measured,by use of a spectrocolorimeter, on a surface of the porous film whichhas not been irradiated with h ultraviolet light having 2 55 W/m²; andWI₁ is WI that is measured, by use of the spectrocolorimeter, on thesurface of the porous film which has been irradiated, for 75 hours, withthe ultraviolet light having 255 W/m².